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I need to replicate 12 Victorian 1.75 inch thick 2 foot x 4 foot ornate triangular porch brackets, plus some corbels and other details. My idea was to glue 2 or 3 layers of marine grade plywood together and have a CNC machine mill them. Then I read the only difference between marine grade plywood and regular plywood is there are no voids, but I'm okay with using some wood putty in voids if it will save me $900.

Will plywood be okay outdoors if I paint it? Or will it fall apart? Should I buy MDO instead? The look i important as well -- will the edges look like wood or like layers of plywood? Is there another option I'm not thinking of?

Difficulty: it's a "historic" home so I can't use any non-historic material like PVC. But it's an 1865 home and plywood was invented in 1865 so technically it should be allowed...

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The plywood will hold up but the edges will look like plywood sooner than later. Keep in mind that the original wood managed to last 150 years and probably failed from lapse of maintenance. I would recommend using solid wood and my first choice would be white oak. I would avoid Teak, though it has excellent rot resistance, it is an oily wood and would present challenges to gluing and painting. With proper maintenance you could just as easily use pine or fir. The key to survival is to keep out the water, water kills houses.

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the only difference between marine grade plywood and regular plywood is there are no voids

This is not true. Marine plywood has glue that is rated for wet conditions, regular plywood has no such requirement. There is no guarantee with non-marine plywood that it will hold up outside. If you decide to go with plywood I would definitely try to find a true marine grade.

will the edges look like wood or like layers of plywood?

Unless you fill and sand the machined edges of plywood will show through paint. It's very hard to have end grain adjacent to face grain and not notice the difference. Then again, I doubt people would be that close to these pieces so it might work out just fine.

My best recommendation would be PVC, but since that is not allowed I would try to use a rot-resistant solid wood like teak, ipe, or black locust (if you can find it). If you have to use a less rot-resistant species make sure you repaint it every year.

  • I meant to compare marine grade plywood with "exterior grade" plywood that has waterproof glue. I know there are a few kinds like MDO and CDX. I considered 2x12 cedar for other parts but I can't find any yet. I imagine teak would cost thousands... The originals must have been constructed in pieces, maybe I can pull one apart and learn some tricks. – Seth Feb 21 '18 at 3:57
  • Yeah, the price of ipe and teak is why I included black locust. It's hard to find milled, but is a very common species domestically. Cedar and white oak are not quite as rot-resistant, but if you repaint every year they should last just fine. – SaSSafraS1232 Feb 21 '18 at 18:49

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